Measurements of spontaneous ion conduction fluctuations in squid axon membrane and in artificial lipid bilayer membranes constitute a significant direct source of informaion regarding the microscopic processes associated with ion movements across these membranes. Spectral analysis of the fluctuations associated with K conduction gives a basis for comparison of kinetic schemes and number of conduction states. Noise induced in axon K channels by quarternary ammonium compounds provides additional information on K conduction and the membrane binding site reaction. Examination of the noise produced by Tl, NH4, and Rb ions as substitutes for K should indicate the extent to which these permeable ions alter channel kinetics. Identification and characterization of Na conduction noise will be pursued. The influence of membrane fluidity on conduction noise will be studied in bilayer membranes.